Rafael Alcalá

The volume holographic optical storage potential in azobenzene containing polymers

Volume holographic data storage is one of the most promising techniques to improve both the storage capacity of devices and the transfer data rate. Among the materials proposed as storage data media, azobenzene containing polymers have received much attention. Some of their properties seem to be suitable for holographic storage applications. However, they still present several problems, mainly those related with light sensitivity, response time and stability of the stored information. In this article we review the work performed on volume holographic storage using azobenzene containing polymers as well as the different approaches that have been proposed to overcome those problems.

The first tetrathiafulvalene derivatives exhibiting second-order NLO properties

Abstract The second order NLO properties of push-pull TTF derivatives have been studied for the first time. The effect of modifying both the acceptor and the donor moieties on the μβ values has been studied by experimental (EFISH) and theoretical techniques.

Second-order nonlinear optical properties of tetrathiafulvalene-π-(thio)barbituric acid chromophores

Abstract Donor-acceptor polyenes that combine an electron-donating tetrathiafulvalene (TTF) unit with a barbituric or thiobarbituric acid have been prepared for the first time and characterized as nonlinear optical (NLO) chromophores. The electrochemical and thermal properties of these derivatives are also reported.

Photoinduced supramolecular chirality in side-chain liquid crystalline azopolymers

A series of liquid crystalline homopolymers that have photoisomerizable cyanoazobenzene groups in the side chain has been synthesized and characterized. Thin films of these polymers have been processed in order to study the absorption spectra and circular dichroism responses after illumination with 488 nm circularly polarized light. The tendency of the azobenzene units to aggregate and the modification of the electronic spectra by illumination were studied by UV-vis spectroscopy. Illumination with circularly polarized light of the azopolymers gave rise to a photoinduced circular dichroism response, which suggests supramolecular structures with a chiral orientation of the azobenzene chromophores in H-aggregates. The photoinduced supramolecular order depends on the polymer structure, i.e., on the Tg and the length of the flexible spacer, and the nature of the liquid crystal phase. Identical absorption spectra and reverse circular dichroism spectra were obtained after irradiation with circularly polarized light of opposite handedness and this is due to the photoinduction of enantiomeric supramolecular structures. The thermal stability above Tg of the photoinduced supramolecular chirality is determined by the polymer structure.

Second-order nonlinear optical properties of tetrathiafulvalene-π-3-(dicyanomethylidene)indan-1-one chromophores

Novel conjugated donor-acceptor chromophores, based on the strong electron donating tetrathiafulvalene moiety and the strong electron-withdrawing 3-(dicyanomethylidene)indan-1-one acceptor, exhibit large second-order optical nonlinearities. The effect of increasing the length of the polyenic spacer on the NLO properties of the new molecules has been studied by using the electric field-induced second harmonic generation (EFISH) method.

Synthesis and characterization of novel NLO-phores from π-extended tetrathiafulvalene (TTF) derivatives

Abstract Donor-π-Acceptor systems containing extended TTF derivatives with quinonoid structures have been synthesized and characterized. Their properties as second order NLO chromophores have been studied by experimental (EFISH) and theoretical methods.

Biphotonic holographic gratings in azobenzene polyesters: Surface relief phenomena and polarization effects

Biphotonic holographic gratings have been recorded in a side-chain azobenzene liquid crystalline polyester using a blue incoherent source and a He–Ne laser. Intensity gratings and the appearance of surface relief have been observed when two linearly polarized beams from a He–Ne laser are made to interfere on a film illuminated with blue light. Polarized holographic gratings are also created with two orthogonally circularly polarized He–Ne beams. All these gratings are stable in darkness but can be erased with blue light.

Synthesis and properties of push–pull chromophores for second-order nonlinear optics derived from π-extended tetrathiafulvalenes (TTFs)

Abstract Novel π-extended tetrathiafulvalene (exTTF) derivatives connected to a p -nitrophenyl electron-accepting unit through a π-conjugated oligoenic spacer ( 14a – c – 16a – c ) show good second-order optical nonlinearities. The effect of the conjugation length of the oligoenic spacer and the presence of a thiophene ring in the π-conjugated bridge ( 20a – c ) have been theoretically (PM3) and/or experimentally (EFISH technique) studied. The redox properties of the novel compounds have been investigated by cyclic voltammetry (CV) and the CV data reveal the good donor ability of the exTTF unit. Although the energy of the intramolecular charge-transfer (ICT) band displayed by these compounds and their redox potentials are fairly insensitive to the nature of the conjugated spacer linking the donor and the acceptor moieties, an increase in their second order nonlinear optical responses is observed on increasing the conjugation lengths. Compounds 20a – c bearing a thiophene ring show a remarkable thermal stability with decomposition temperatures over 400°C. A new series of D-π-A compounds ( 23a – c ) bearing a 1,3-dithiol-2-ylidene donor unit has also been synthesized and studied.

Synthesis and characterization of reactive liquid crystals and polymers based on terphenyl derivatives

Abstract A series of liquid crystalline methacrylates based on terphenyl derivatives bearing a donor–π-acceptor structure has been synthesized. The mesomorphic, optical absorption, photoluminescent and non-linear optical properties of the monomers have been investigated. Polymerization of these monomers was first carried out in solution but the dicyanovinyl group seems to inhibit the reaction despite the fact that the DSC curves of the monomer provide evidence for the occurrence of thermal polymerization. An IR study of this thermal polymerization confirms that only methacrylate groups are involved in the polymerization process. The mesomorphic and thermal properties of the resulting side-chain polymers have also been investigated.

BETA -DIKETONE, PYRAZOLE AND ISOXAZOLE DERIVATIVES WITH POLAR GROUPS : LIQUID CRYSTALLINE AND NON-LINEAR OPTICAL PROPERTIES

The liquid crystalline behaviour of 1-(4-n-decyloxyphenyl)-3-(4-X-phenyl)propan-1,3-diones and their pyrazole and isoxazole derivatives has been studied by optical microscopy, DSC and X-ray techniques. The 4-substituents (X) were chosen to include a range of different polar and non-polar substituents: H, OCH3, Cl, Br and CN. A monotropic SA phase is observed for the beta-diketone derivative in which X CN and this is the first example of this phase found in a 1,3-diphenylpropan-1,3-dione derivative. The majority of the pyrazole and isoxazole compounds show SA phases. As regards the cyano-substituted compounds, X-ray diffraction studies on the mesophase show that the layer spacing is consistent with a partial bilayer SA mesophase. The first hyperpolarizabilities of the cyano-derivatives due to their push-pull structure have been measured by the EFISH method. Values for these compounds were found to be comparable to those for other conjugated CH3O-pi-CN systems.